Examples of such adaptations include increased mitochondria[Define] density and glycogen storage. These adaptations increase the maximal rate at which skeletal muscle can produce ATP and therefore sustain exercise.

It is believed that an enzyme called 5' adensine monophosphate-activated protein kinase (AMPK) plays a very important role in the adaptations seen from exercise. Maintaining optimal levels of ATP is of prime importance for a cell's survival, as ATP is its source of energy.

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AMPK is a metabolic-stress-sensing protein kinase; meaning it functions as a cellular fuel gauge. This enzyme serves to maintain cellular energy homeostasis, specifically during times of stress caused by exercise or nutrient intake (diet). The activation of AMPK initiates signaling cascades that stimulate changes in glucose, fatty acid metabolism, and gene expression, which ultimately results in an increased ability to produce ATP.

The molecule of ATP, referred to as a "high-energy phosphate", is made up of adenine and ribose (adenosine) bonded to three phosphates (Pi- phosphorus and oxygen). The energy stored in ATP is held in the two outermost phosphate bonds. These outermost bonds are referred to as "high-energy bonds."

When water joins with ATP, catalyzed by the enzyme ATPase, the outermost phosphate bond is cleaved, producing adenosine diphosphate (ADP) and a phosphate ion as well as liberating 7.3 kcal of free energy to be used for work. ADP levels increase as ATP is used for energy.

The body uses various energetic pathways to maintain cellular ATP levels, such as the glycolysis[Define] and oxidative phosphorylation. Two enzymes are responsible for maintaining ATP levels as soon as muscle contraction begins; more precisely as soon as the muscle starts using ATP at an accelerated rate.

The first enzyme is myokinase, also known as adenylate kinase, which catalyzes the reaction in which a phosphate is transferred from one ADP molecule to another ADP molecule, creating one ATP and one AMP molecule:

ADP + ADP ATP + AMP

The other enzyme is creatine phosphokinase, which catalyzes the reaction in which a phosphate is transferred from phosphocreatine (PCr) to ADP to form one ATP and creatine (Cr) molecule:

PCr + ADP ATP + Cr

During exercise, AMP levels increase and PCr decreases in the working muscle, both of which signal a need to produce more ATP.

AMPK is activated by any stress that inhibits ATP production or increases ATP consumption15. This includes hypoxia, heat shock, exercise, and glucose deprivation. AMPK is also activated by the hormones leptin and adiponectin15. As its name suggests, AMP directly activates AMPK.

Specifically, AMPK is activated when there is an increase in the AMP/ATP or creatine/phosphocreatine ratio, or more simply, an energy deficit1. AMP can directly activate AMPK, but can also bind to AMPK causing it to become a more efficient substrate (the substance acted upon by an enzyme) for the upstream AMPK kinase (AMPKK); AMP can also activate AMPKK directly2. AMPKK activates AMPK by phosphorylation (adding a phosphate).

Phosphocreatine serves as an inhibitor of AMPK activation; therefore decreased PCr levels can cause AMPK activation2. Increased levels of muscle glycogen also inhibit AMPK1, as sensed by a glycogen-binding domain on the Â subunit of AMPK.

It is theorized that this glycogen-binding domain serves as a sensor of glycogen levels15. As mentioned, exercise (muscle contraction) causes AMP levels to increase, PCr levels to decrease, and depletion of muscle glycogen and has been proven to activate AMPK2.

AMPK can also be experimentally activated by the nucleoside analogue 5-amino-4-imidazolecarboxamide riboside (AICAR) and Â-guanadionopropionic acid (Â-GPA). Once AICAR enters a muscle cell, it is metabolized into ZMP, an analogue of AMP. Â-GPA, is a creatine analogue which depletes cellular phophsocreatine and ATP3. Both compounds create an energy-stress signal and thereby activate AMPK.

When AMPK is activated, ATP-consuming pathways, such as protein and fat synthesis, are shut off and ATP-producing pathways, such as fatty acid oxidation, are turned on in order to spare ATP as well as restore diminished levels. AMPK accomplishes this by direct phosphorylation of regulatory proteins and by indirect effects on gene expression14.

Glucose enters muscle cells by being transported in by the glucose transporter GLUT-4. During the basal state (no contraction/low insulin) the GLUT-4 transporters lie beneath the muscle cell's sarcolemma and T-tubule membranes and cannot transport any glucose.

Contraction and insulin can independently and additively cause the GLUT-4 transporters to translocation from below the membranes and insert into the membrane where they can now transport glucose into the muscle cell. It is believed that AMPK plays a part in the contraction-induced translocation of GLUT-4 transporters1 and has been shown to increase skeletal muscle glucose uptake4, 5.

AMPK has also been shown to increase GLUT-4 mRNA gene transcription, which would increase the total number of receptors available to transport glucose15.

When insulin was added with AICAR treatment, glucose uptake was increased above AMPK activation alone1. When AMPK activation by AICAR was coupled with muscle contraction, no further increase in glucose uptake was seen, which suggest AMPK and contract share a common mechanism of action1.

An interesting note is while AMPK turns off anabolic pathways and turns on catabolic, ATP producing pathways, AICAR treated rats have been shown to possess increased glycogen stores1. AMPK has an inhibitory effect on glycogen synthase, the enzyme used in glycogen storage, yet glycogen storage is not diminished, but rather enhanced.

AMPK activation has been shown to decrease glucose oxidation rates by 50% in rats6. Because of this and other findings, it is proposed that AMPK's action in restoring ATP levels in mainly accomplished by promoting fatty acid oxidation over glucose oxidation.

Fatty Acid Oxidation

Beta-oxidation of fatty acids (in the form of fatty acyl-CoA) in the mitochondria supplies a significant source of ATP regeneration during exercise, specifically endurance-type exercise. A protein called carnitine palmitoyltransferase-1 (CPT-1), which is found on the outer membrane of mitochondria, regulates fatty acid oxidation.

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CPT-1 carries fatty acids across the membrane into the mitochondria by binding to the acyl end of the fatty acyl-CoA. CPT-1 is in turn regulated by malonyl-CoA, which is synthesized by acetyl-CoA carboxylase (ACC) in skeletal muscle. Malonyl-CoA prevents fatty acid oxidation by competitively binding with CPT-1, which prevents fatty acids from binding to it. Malonyl-CoA concentrations are lowered by malonyl-CoA decarboxylase.

In order to take the brakes off fatty acid oxidation, malonyl-CoA levels must be decreased; inhibiting ACC or activating malonyl-CoA decarboxylase can accomplish this.

Activation of AMPK, by exercise, electrical stimulation, and AICAR, has also been shown to activate malonyl-CoA decarboxylase1. The combination of deactivated ACC and activated malonyl-CoA decarboxylase supports AMPK's roll in fatty acid oxidation in skeletal muscle.

The above would agree with AMPK's proposed action of increasing fatty acid oxidation while decreasing glucose oxidation since increased fatty acid oxidation would suppress glucose oxidation at the PFK-1 and pyruvate dehydrogenase level10. This is supported by in-vitro findings, which showed AICAR to increase fatty acid oxidation and inhibit glucose oxidation simultaneously11.

Chronic AMPK activation by AICAR has been shown to cause the same adaptations as well as increased protein expression of the following mitochondrial enzymes: malate dehydrogenase, citrate synthase, cytochrome C, hexokinase II, succinate dehydrogenase, d-aminoevulinate synthase, and uncoupling protein-3 (UCP-3)12. All of these enzymes are involved in fatty acid oxidation.

To further support the idea that glucose metabolism is secondary to fatty acid oxidation, no glycolytic enzymes have been found to be up regulated by AMPK activation1.

AMPK has been shown to play a role with mitochondria biogenesis in skeletal muscle. A study using Â-GPA injections on mice to induce an energy-deprived state found that GPA caused a significant increase in mitochondrial DNA and density in skeletal muscle in wild mice (normally functioning AMPK), but not in mice expressing an inactive dominant-negative AMPK, which shows AMPK is essential component of mitochondria biogenesis13.

An increased number of mitochondria would allow more total fatty acids to be oxidated by increasing the body's capacity to do so.

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Summary And Application

AMPK serves to make one's body more efficient in producing ATP. It does this by increasing glucose uptake (GLUT-4 translocation and gene expression) and insulin sensitivity, promoting fatty acid oxidation and mitochondria biogenesis, as well as a host of other effects. The adaptations caused by AMPK make the body more efficient in producing ATP and therefore survival.

The easiest and most economical way to activate AMPK is through exercise, but supplement induced activation may prove useful for those looking to lose weight or improve different aspects of overall health. An example of such a supplement is Avant Labs' Phenogen. Phenogen targets AMPK to aid in fat loss by use of Â-GPA. Phenogen also contains SesaThin and Salvia Miltiorrhiza, which will both enhance fat loss.

One potential roadblock with AICAR is it does not mimic all the changes seen in high-energy phosphate concentrations seen during exercise2. ATP and PCr levels are not affected and ZTP has been shown to increase, which would interfere with full activation of AMPK12. New substances that activate AMPK will be researched. Conclusion: much more research on AMPK is needed to fully understand its applications and the most effective methods, besides exercise and diet, to activate it.